1. Field of the Invention
The present invention relates to a power supply, and more specifically relates to a control method of a switching mode power supply.
2. Description of Related Art
Power supplies have been used to convert an unregulated power source to a regulated voltage or current. FIG. 1 shows a traditional power supply. The power supply comprises a PWM controller 10 for generating a switching signal VPWM. The switching signal VPWM is used for switching a transformer 11 via a transistor 20. The duty cycle of the switching signal VPWM determines the power delivered from an input of a power source to an output of the power supply. Although the switching technology can reduce the size of the power supply, switching devices generate electric and magnetic interference (EMI) that interferes the power source. An EMI filter 15 equipped at an input of the power supply is utilized to reduce the EMI. However, the EMI filter 15 causes power consumption and increases the cost and the size of the power supply. In recent development, many prior arts have been proposed to reduce the EMI using frequency jitter. For example, “Effects of Switching Frequency Modulation on EMI Performance of a Converter Using Spread Spectrum Approach” by M. Rahkala, T. Suntio, K. Kalliomaki, APEC 2002 (Applied Power Electronics Conference and Exposition, 2002), 17-Annual, IEEE, Volume 1, 10–14, Mar., 2002, Pages: 93–99; “Offline Converter with Integrated Softstart and Frequency Jitter” by Balu Balakirshnan, Alex Djenguerian, U.S. Pat. No. 6,229,366, May 8, 2001; and “Frequency Jittering Control for Varying the Switching Frequency of a Power Supply” by Balu Balakirshnan, Alex Djenguerian, U.S. Pat. No. 6,249,876, Jun. 19, 2001. However, the disadvantage of these prior arts is that the frequency jitter generates an undesirable ripple signal at the output of the power supply. The undesirable ripple signal generated by the frequency jitter could be realized as following description. An output power PO of the power supply is the product of an output voltage VO and an output current IO, which is given by,PO=Vo×Io=η×PIN . . .   (1)
An input power PIN of the transformer 11 and a switching current IP can be respectively expressed by,
                              P          IN                =                              1                          2              ×              T                                ×                      L            P                    ×                      I            P            2                                                            I          P                =                                            V              IN                                      L              P                                ×                      T            ON                              Where η is the efficiency of the transformer 11; VIN is an input voltage of the transformer 11; LP is the primary inductance of the transformer 11; T is a switching period of the switching signal VPWM; TON is an on-time of the switching signal VPWM.The equation (1) can be rewritten as,
                              P          O                =                  η          ×                                                    V                IN                2                            ×                              T                ON                2                                                    2              ×                              L                P                            ×              T                                                          (        2        )            
The switching period T varies in response to the frequency jitter. As shown in equation (2), the output power PO varies whenever the switching period T varies. And therefore the undesirable ripple signal will be generated as the output power PO varies.
An object of the present invention is to provide a frequency jitter to reduce the EMI for a power supply, in which the frequency jitter of the present invention will not generate the undesirable ripple signal at the output of the power supply.